In general, friction materials for brakes are typically produced by combining together asbestos as a strength material, ceramic (inorganic) powders such as oxides and carbides as a friction coefficient (hereafter referred to as ".mu.")-increasing material, and graphite, soft metal, etc. as a friction-controlling material by the use of an organic binder (resin). Asbestos, however, has well known safety and hygienic disadvantages in the production or use thereof. Accordingly, it has been proposed to make friction materials for vehicle brakes asbestos-free by using steel fibers as the strength material in place of asbestos (see for example, Japanese Patent Application (CPI) Nos. 157673/80 and 111373/82 which disclose friction materials using iron powder and steel fibers in place of asbestos). Thus, the use of steel fibers in friction materials is, in and of itself, known.
It is necessary for the friction material however to exhibit a high friction coefficient (.mu.). Accordingly, for those friction materials using conventional steel fibers, various techniques are known to increase the friction material's friction coefficient. For example, the friction coefficient is increased by increasing the carbon content of the steel fibers so as to also increase the hardness of the resulting friction material. Additionally, the diameter of the steel fibers can be increased to thereby increase the friction coefficient as a result of its increased abrasive surface area with respect to the bearing surface of the brake drum, rotor or the like. Such a technique leads to a problem that the material of the brake drum etc. is seriously worn.
A friction material using steel fibers has thus been needed which has a high friction coefficient while yet decreases the wear amount of the material of the brake drum, rotor and the like against which the friction material bears.
In general, P (phosphorus) and S (sulfur) are impurities for steel, and are responsible for a reduction in its strength, toughness, workability, and so forth. Thus, it has been conventional wisdom in the production of steel, to decrease the phosphorus and sulfur contents as much as possible and such has been the case for steel fibers to be used in brake friction materials. In all conventional steel fibers, both the P and S contents are typically not more than 0.04% while the fiber's carbon content is between 0.1 to 0.2%.
It has been discovered, according to the present invention, that larger amounts of P and S in the steel fibers than have been conventionally utilized surprisingly increase the friction coefficient (.mu.) while at the same time, decrease the wear of the brake drum or rotor material. That is, as the P and S contents increase, the friction coefficient (.mu.) is also increased and the material against which the friction material bears is less worn as compared to conventional friction materials for similar purposes.
The present invention thus overcomes the above defects of the conventional friction materials using conventional steel fibers and provides a friction material having a high friction coefficient and less abrasion of the material against which the friction material bears by using steel fibers containing at least 0.07% of phosphorus, at least 0.10% of sulfur, or at least 0.15% of both phosphorus and sulfur.